From: Shaoxuan Yuan <shaoxuan.yuan02@gmail.com>
Date: Thu, 01 Sep 2022 22:36:20 GMT
Subject: Re: [PATCH v3 3/3] builtin/grep.c: walking tree instead of expanding index with --sparse
Message-ID: <cd11cc71-397a-e186-7d53-4a122f830903@gmail.com>
In-Reply-To: <xmqqpmgf9fpr.fsf@gitster.g>

On 9/1/2022 10:17 AM, Junio C Hamano wrote:
 > Shaoxuan Yuan <shaoxuan.yuan02@gmail.com> writes:
 >
 >> Before this patch, whenever --sparse is used, `git-grep` utilizes the
 >> ensure_full_index() method to expand the index and search all the
 >> entries. Because this method requires walking all the trees and
 >> constructing the index, it is the slow part within the whole command.
 >>
 >> To achieve better performance, this patch uses grep_tree() to search the
 >> sparse directory entries and get rid of the ensure_full_index() method.
 >
 > When you encounter a "sparsedir" (i.e. a tree recorded in index),
 > you should know the path leading to that directory. Even though I no
 > longer remember the details of the implementations of grep_$where()
 > which I did long time ago, I think grep_tree() should know how to
 > pass the leading path down, as that is the most natural way to
 > implement the recursive behaviour.  This patch should be able to
 > piggyback on that.

Yes, though this commit [1] from 6 years ago started to assume that
grep_tree() only accepts root tree or commit, so the function fails
to process a tree like "sparsedir". It's the pathspec matching base that
was messed up. The support for a tree that is not at root-level was
added in this series.

[1] 74ed43711fd1cd7ce155d338f87ebe52cb74d9e2

 >> @@ -537,8 +534,26 @@ static int grep_cache(struct grep_opt *opt,
 >>
 >>          strbuf_setlen(&name, name_base_len);
 >>          strbuf_addstr(&name, ce->name);
 >> +        if (S_ISSPARSEDIR(ce->ce_mode)) {
 >> +            enum object_type type;
 >> +            struct tree_desc tree;
 >> +            void *data;
 >> +            unsigned long size;
 >> +            struct strbuf base = STRBUF_INIT;
 >> +
 >> +            strbuf_addstr(&base, ce->name);
 >> +
 >> +            data = read_object_file(&ce->oid, &type, &size);
 >> +            init_tree_desc(&tree, data, size);
 >>
 >> +            /*
 >> +             * sneak in the ce_mode using check_attr parameter
 >> +             */
 >> +            hit |= grep_tree(opt, pathspec, &tree, &base,
 >> +                     base.len, ce->ce_mode);
 >
 > OK.  Instead of inventing a new "base" strbuf, we could reuse
 > existing name while running the grep_tree() and restore it after it
 > returns, and I suspect that the end result would be more in line
 > with how grep_cache() uses that "name" buffer for all the cache
 > entries.  But that is not a correctness issue (it is move about
 > preventing from making the code worse).

Oh right, thanks for the suggestion!

 >> @@ -598,7 +613,14 @@ static int grep_tree(struct grep_opt *opt, 
const struct pathspec *pathspec,
 >>          int te_len = tree_entry_len(&entry);
 >>
 >>          if (match != all_entries_interesting) {
 >> -            strbuf_addstr(&name, base->buf + tn_len);
 >> +            if (S_ISSPARSEDIR(check_attr)) {
 >> +                // object is a sparse directory entry
 >
 > No // comments, please.

OK.

Thanks,
Shaoxuan